1. Field of the Invention
The present invention relates to a method of fabricating a liquid crystal display panel by patterning a silicon film that is crystallized by sequential lateral solidification.
2. Discussion of the Related Art
Using the fact that a silicon grain grows in a vertical direction at an interface between a liquid phase silicon region and a solid phase silicon region, sequential lateral solidification (hereinafter referred to as xe2x80x9cSLSxe2x80x9d) is a technique of crystallizing an amorphous silicon film by enhancing the size of the silicon grain. SLS is achieved by growing the silicon grain laterally to a predetermined length by manipulating the displacement of the energy and irradiation range of a laser beam (Robert S. Sposilli, M. A. Crowder, and James S. Im, Mat. Res. Soc. Symp. Proc., Vol. 452, 956957, 1997). Sequential lateral solidification can be applied to the field of laser crystallization.
Conventional silicon film crystallization using SLS is explained in accordance with the FIGS. 1A-4. In FIGS. 1A-1C, there is shown schematically the process of crystallizing an amorphous silicon layer by SLS, illustrating the respective growing states of silicon grains in accordance with the number n of laser irradiations.
Referring first to FIG. 1A, a bar-type laser beam (a laser beam with a cross-sectional shape in the shape of a bar) with a predetermined width is irradiated on amorphous silicon film 11. The laser is supplied with enough energy to melt the entire area of the amorphous silicon film that is irradiated by the laser beam (i.e., the first irradiation region). The irradiated area begins to solidify as soon as the laser beam melts it. During solidification, silicon grains 11G and 11Gxe2x80x2 grow laterally from interfaces 11L and 11Lxe2x80x2. Interfaces 11L and 11Lxe2x80x2 are the boundaries between the section of the silicon film that is liquid as a result of irradiation by the laser beam (i.e., the first irradiation region) and the rest of the silicon film that is not irradiated by the laser beam and that remains solid. The growth length of the silicon grain by a single laser irradiation (or xe2x80x9claser pulsexe2x80x9d) depends on the thickness of a silicon film and the magnitude of laser energy. When a width W of the melted silicon section (i.e., the first irradiation region) is shorter than twice the growth length [of the silicon grain by one laser pulse], as shown in FIG. 1A, silicon grains 11G and 11Gxe2x80x2 stop growing as soon as they collide in the center of the melted silicon section. Grain boundary 10a is generated from the collision of the silicon grains 11G and 11Gxe2x80x2.
On the other hand, the silicon grains stop growing when the width of the melted silicon section is longer than twice the growth length [of the silicon grain by one laser pulse]. This is because the silicon grains growing from both interfaces collide with micro-silicon grains generated from the central part of the melted silicon section as the melted silicon cools down.
Referring now to FIG. 1B, a second laser beam of predetermined width W is irradiated on the amorphous silicon film after the film has been moved a distance shorter than the growth length of the silicon grain by one laser pulse. The laser beam is supplied with enough energy to melt the entire silicon section that is irradiated by the laser beam (i.e., the second irradiation region). The second irradiation region melts and begins to solidify immediately as previously described. During solidification, silicon grains 11G and 12G grow laterally from interfaces 12L and 12Lxe2x80x2. Silicon grain 11G, which is formed by the first laser beam irradiation, works as a seed for crystallization as it continues to grow laterally. Silicon grains 11G grow in the same direction of the dislocation of the laser beam. Grain boundary 10b is generated from the collision of silicon grains 11G and 12G grown laterally from interfaces 12L and 12Lxe2x80x2.
Referring now to FIG. 1C, the desired length of silicon grain 11G is attained by repeating n times the silicon crystallization process comprising the steps of moving the amorphous silicon film (or the laser beam) by an amount no greater than twice the growth length, melting the silicon film by laser beam irradiation, and solidifying the melted silicon film. Silicon grain 11G grows laterally in the direction of laser scanning from the origin of formation.
SLS may be used to greatly increase the size of a silicon grain. When SLS is applied to a silicon film of large scale, silicon crystallization is achieved by applying a plurality of laser beams to the silicon film simultaneously to improve product yield.
FIG. 2 shows a mask for crystallizing a silicon film of large scale according to a related art technique. A mask may be used for patterning a single laser beam into a plurality of laser beams. Mask 20 has a plurality of bar-shaped, ray-penetrating regions 20-2. When mask 20 is used, a plurality of laser beams, the number and shapes of which are the same as regions 20-2, is provided.
FIG. 3 shows polycrystalline silicon film 30 of large scale formed by the related art technique. In FIG. 3, polycrystalline silicon film 30 is attained via SLS silicon crystallization using a plurality of bar-shaped laser beams that are provided by the mask shown in FIG. 2. Polycrystalline silicon film 30 comprises silicon grains G. Silicon grains G, which are grown along a first direction x, form grain boundaries 32 between the silicon grains. Boundary 31 is the boundary between the polycrystalline silicon regions formed by a single laser beam.
FIG. 4 shows the channel directions of thin film transistor (TFT) devices 41 and fabricated 42 using the method of fabricating an LCD panel from polycrystalline silicon film 30 according to the related art (i.e., growing silicon grains G in a direction horizontal to a substrate). Referring to FIG. 4, LCD devices comprising an LCD driver and an LCD pixel have channel directions that are horizontal x or perpendicular y to the substrate. Device 41, in which the channel direction is horizontal x, has the same characteristics as a single crystalline silicon device because device 41 contains only one grain boundary 32 to obstruct movement of charge carriers. On the other hand, numerous silicon grain boundaries 32 exist on the carrier paths in device 42 in which the channel direction is perpendicular y to the substrate. The result is that the characteristics of device 42 are inferior to those of device 41 because device 42 is not a single crystalline device.
The electrical irregularity of devices fabricated using this method result in LCD panels with unreliable drivers and pixel parts. A need exists, therefore, for a method of fabricating an LCD panel that provides both a reliable driver TFT and a reliable pixel TFT.
Accordingly, the present invention is directed to a method of fabricating a liquid crystal display panel that substantially obviates one or more of the problems, limitations and disadvantages of the related art.
An object of the present invention is to provide a method of fabricating a liquid crystal display panel that provides a driver and a pixel part comprising TFT devices having improved and uniform electric characteristics.
Additional features and advantages of the invention will be set forth in the description that follows and, in part, will be apparent from the description or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof, as well as the appended drawings.
To achieve these and other advantages, and in accordance with the purpose of the present invention as embodied and broadly described, the present invention provides a method of fabricating a liquid crystal display comprising the steps of preparing a silicon film, crystallizing the silicon film by growing silicon grains on a slant relative to the horizontal direction of the silicon film and forming TFT devices in which channels are arranged in a horizontal direction or a perpendicular direction with respect to the silicon film.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.